Night Watch - Still Terry's best, IMO.
Unseen Academicals - Runs Night Watch really close.
Carpe Jugulum - Granny's speech to Death about what it means to live at the edges of things would be enough in itself, but the fact that the rest of the book is so excellent as well means it's well worth its place.
The Last Continent - For me, Terry's outright funniest book. The scene with the Drop Bears is sublimely silly and one of the funniest things I've ever read.
The Colour of Magic - As a huge fan of Fritz Leiber, Robert E. Howard and Swords & Sorcery in general this always makes me smile.
* Correction - Carpio diem